Silicide layers of sub-micron (i.e. less than 1 micron) dimensions are conventionally used in integrated circuits. Such layers can be formed in a two step process as follows. In a first step, a film of metal (such as titanium, cobalt or nickel) is deposited onto a silicon substrate, or a layer of polysilicon. In a second step, the film is annealed to form a metal-silicon compound (such as titanium silicide, cobalt silicide, or nickel silicide). Conventionally, the sheet resistance of such a silicide layer is measured on a test wafer, and the measured sheet resistance may be multiplied by a measurement of a silicide layer's thickness on a production wafer, to determine a property of the layer, called “resistivity.”
A number of methods exist for measuring sheet resistance of the test wafer. In one method, four probes are brought into physical contact with the silicide layer, to measure the layer's sheet resistance directly. See, for example, “The Four-Point Probe”, Section 1.2, pages 2-20 in the book “Semiconductor Material and Device Characterization” by Dieter K. Schroder, John Wiley & Sons, Inc., New York, 1990. The just-described method requires the silicide layer to have an area (e.g., 5 square mm) that may be several orders of magnitude larger than the area of a silicide layer after etching (e.g., <0.5 square microns). Due to such a requirement on the size of the silicide layer (to have a 108 times larger area), and the need to contact the silicide layer, such measurements are performed prior to patterning (i.e. typically on a test wafer).
Many methods, such as spectroscopic ellipsometry, Rutherford backscattering (RBS), scanning electron microscopy and energy dispersive x-ray spectrometry can also be used to determine the sheet resistance, as described in, for example, an article entitled “Spectroscopic ellipsometry investigation of nickel silicide formation by rapid thermal process” by Yaozhi Hu and Sing Pin Tay, Journal of Vacuum Science Technology, volume 16, no. 3, published May/June 1998 by the American Vacuum Society.
U.S. Pat. No. 5,228,776 granted to Smith et al. (hereinafter “Smith”) describes measuring changes in optical reflectivity (column 4, line 5-6) caused by thermal waves (column 3, line 42) to “monitor variations in electrical conductivity and resistance . . . ” (column 4, lines 53-54). Specifically, Smith requires “periodically exciting the sample at a highly localized spot on the sample surface . . . The pump beam functions to periodically heat the sample which in turn generates thermal waves that propagate from the irradiated spot . . . Features at or beneath the sample surface can be studied by monitoring the variations they induce in these waves” (column 1, lines 25-40). Note also that Smith requires the pump and probe beams to be non-coincident and non-coplanar.
Smith also states that “when the optical reflectivity of the sample is to be monitored, it is desirable to arrange the pump and probe beams to be coincident on the sample” (column 1, lines 60-64). When using such coincident beams, Smith notes problems created by “surfaces associated with defective vias are often not optically flat . . . ” (column 3, lines 6-13). Moreover, prior art also states that “[w]hen materials other than semiconductors are to be evaluated, such as metals . . . analysis of the thermal wave patterns is required” (see U.S. Pat. No. 4,854,710 at column 7, lines 41-44).
See also U.S. Pat. No. 5,978,074 granted to Opsal et al.